Common Eider (Somateria mollissima)

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Image of a Common Eider in its natural habitat
Birds Eider

Introduction

The Common Eider, Somateria mollissima, is a large sea duck of northern coastal waters. It breeds around Arctic and sub-Arctic shorelines and islands, and many populations migrate or disperse to ice-free coastal waters in winter. Its specialised bill, dense plumage and substantial body help it forage and survive in cold marine environments.

Physical Description

The Common Eider is a bulky, broad-bodied duck measuring approximately 50–71 cm in length. Adult males are distinctive in breeding plumage, with a mostly white back and breast, black flanks and belly, a black crown, and pale green colouring on the nape and sides of the neck. The bill is broad and wedge-shaped, with enlarged feathered structures at its base.

Females are smaller and brown overall, with fine dark barring that provides camouflage while nesting. Their plumage varies geographically, and some populations have greyer or more reddish-brown females. Juveniles resemble females, while first-winter and non-breeding males are generally brown with variable white feathering before acquiring adult male plumage.

Six subspecies are commonly recognised in current taxonomic treatments, although the limits and treatment of some geographical forms have varied. Named forms include the nominate S. m. mollissima, S. m. faeroeensis, S. m. borealis, S. m. sedentaria, S. m. dresseri and S. m. v-nigrum.

Range and Habitat

Common Eiders occur around the northern coasts of Europe, Asia and North America. Their breeding range includes Arctic and sub-Arctic regions from eastern Siberia across the North Atlantic to Greenland and Iceland, as well as northern and north-eastern Canada and the north-eastern United States. In winter, many populations move southwards to sheltered coastal waters, although some remain near breeding areas where open water is available.

The species is strongly associated with marine habitats, especially rocky coastlines, islands, islets, tidal channels and shallow waters over productive seabeds. Breeding birds nest on low coastal islands, shorelines and tundra, often close to the sea. Some also use coastal ponds, lagoons, gravel spits, heathland and stunted vegetation. Wintering flocks commonly gather where strong tides and rocky substrates support abundant shellfish.

Geographical variation is pronounced. The Pacific, Atlantic, northern, Hudson Bay, European and Faroe Island forms differ in size, bill structure, plumage and distribution, although these differences do not alter the species’ overall identification in most field situations.

Behaviour and Diet

Common Eiders are powerful divers. They swim underwater using their wings and feet to search for prey, often in shallow coastal waters. Their main food consists of marine invertebrates, including mussels and other molluscs, crustaceans, sea urchins, marine worms and, occasionally, fish eggs. Ducklings also take small crustaceans, gastropods and some marine algae.

The broad bill is well suited to gripping and removing shellfish from rocks and the seabed. Small prey may be swallowed underwater, while larger items such as mussels and sea urchins are often brought to the surface for handling. Feeding activity may increase around productive tidal areas, particularly when prey is accessible at lower tide levels.

Outside the breeding season, Common Eiders are sociable and may form large flocks. Males give soft, cooing calls during courtship, and courtship behaviour can continue within wintering flocks. Females may gather with several broods into crèches, in which ducklings from different mothers remain together under the supervision of one or more females.

Breeding usually begins in late spring or early summer, depending on latitude. The female makes a shallow ground nest near the coast and lines it with down plucked from her breast. A clutch generally contains three to eight eggs. Incubation lasts about 24–26 days, during which the female relies heavily on stored fat reserves and leaves the nest only briefly.

The chicks are precocial. They are covered in down, leave the nest soon after hatching and can feed themselves, although the female leads and protects them for several weeks. True eider down has excellent insulating properties and has traditionally been collected from abandoned nests in some regions.

Identification Tips

  • Look for a large, heavy-bodied sea duck with a long, sloping forehead and a broad bill.
  • Adult males are usually unmistakable: white on the back and breast contrasts with black flanks, belly and crown, while the nape and sides of the neck appear pale green.
  • Females are rich brown or greyish brown with dense dark barring. Their large size, sloping forehead and broad bill help separate them from many other brown ducks.
  • Immature and non-breeding males can look brown and mottled, so plumage should be assessed together with body shape, bill profile and habitat.
  • King Eider is the most important potential confusion species in the northern part of the range. King Eider has a rounder, more block-shaped head, a different bill profile and, in males, a black back rather than the Common Eider’s white back.
  • Spectacled Eider can be distinguished by its conspicuous pale eye patches. Common Eiders lack these patches.

Conservation Status

The Common Eider is listed as Least Concern on the BirdLife International assessment used for the IUCN Red List. This global category does not mean that all populations are secure. Local and regional declines have been documented, and some populations have additional national or regional conservation designations.

Important threats include nest predation, disturbance at breeding colonies, coastal development, pollution and oil spills. Changes in sea-ice conditions, marine productivity and the distribution of shellfish may also affect different populations in different ways. Hunting, egg collection and other forms of exploitation are regulated in many areas, but management requirements vary across the species’ extensive range.

Conservation measures include protecting breeding islands and coastal feeding areas, maintaining marine protected areas, monitoring population trends, regulating hunting and reducing disturbance during the breeding season. International cooperation is particularly important because many populations cross national boundaries during migration.

Frequently Asked Questions

Where does the Common Eider live?

It lives mainly in marine environments around the northern coasts of Europe, Asia and North America. It breeds on coastal islands, shorelines and tundra, and winters in ice-free coastal waters where shellfish are abundant.

What does the Common Eider eat?

Its diet consists mainly of marine invertebrates, especially mussels and other molluscs. It also eats crustaceans, sea urchins, marine worms, fish eggs and, particularly when young, small gastropods, amphipods and some algae.

How can a Common Eider be identified?

The species is a large, bulky sea duck with a long sloping forehead and broad bill. Males are black and white with pale green on the neck, while females are brown with dense dark barring.

Do Common Eiders migrate?

Many populations migrate or disperse between northern breeding grounds and more southerly wintering areas. Others remain near their breeding range if open water is available throughout winter.

How many eggs does a Common Eider lay?

A female generally lays three to eight eggs. She incubates them for approximately 24–26 days and lines the nest with down plucked from her breast.

Are Common Eiders endangered?

The species is currently assessed globally as Least Concern, but some regional populations are declining or face substantial local threats. Conservation status should therefore be considered at both global and regional levels.

References

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